MVR Evaporator

Evaporation Technology Using MVR (Mechanical Vapour Recompression)

Evaporation technology involves separation of Solvent / Water using thermal energy.

To economize the thermal energy initially, Evaporators were used as single stage evaporator later to multistage evaporator say up to 6 – 7 Stages. As 1 Kg/Hr of steam produces 1,2,3,4,5 Kg/hr of vapor from respective effect of  1,2,3,4,5 stages. Thus increasing thermal efficiency of Evaporator with increase in number of effects.

The vapor produced in 1st effect is used as heating steam in second effect. This principle is used for several effects to further improve the steam saving. The maximum heating temperature of the first effect and the lowest boiling temperature of the last effect result in net temperature difference, which is distributed over individual stage or effect.

Further steam consumption is reduced by using thermal vapor recompression (TVR). For 5 effect specific heat consumption is 0.11 per kg Water evaporation.

Finally, use of mechanical vapor recompression system results in further optimization in thermal energy of the system.

The increase in effect increases capital cost significantly but reducing the energy cost considerably.

MVR recompresses the total vapour of an Evaporator effect from a lower temperature/ Pressure to higher temperature or pressure thus heat energy is reused for heating and no vapour needs to be condensed. Only drive electrical energy is used for MVR.

The cost of energy requirement is often lower than steam heated plant, only small quantity of steam is required to balance the total heat requirement including startup load in MVR. Hence only small portion of heat needs to be dissipitated through the cooler.




A Process & MVR Brief Description

Types of constriction of MVR

In evaporation plants, the three types are used:

  • Centrifugal fans
  • Centrifugal (turbo) compressors
  • Rotary (roots) blowers

Centrifugal Fans

Centrifugal fans are continuous-flow machines consisting of a spiral-type housing which accommodates a rotating impeller with radially ending blades or with backward-bent blades.

Centrifugal (turbo) compressor

Centrifugal compressors are also continuous-flow machines in radial construction with an energy transfer comparable to that of a centrifugal fan. Centrifugal compressors also consist of spiral-type housing with axial suction nozzle, accommodating a rotating, usually semi-open impeller.

Rotary Blower

In a Rotary Blower or, Roots Blower, two, two-or three-blade rotary pistons rotate in opposite directions. In a simple design, a rotary piston has a symmetrical, octagonal shape. Together with the circumferential housing, it builds a so-called conveying chamber in which the vapour is conveyed from the suction side to the pressure side of the machine.

B  Principles of Function and Design Features

Monitoring and Safety equipment

Optimum operation of an evaporation plant requires safe and sufficient heating. In plants with mechanical vapour recompression, the permanently reliable operation of the compressor is vital.

Monitoring and Safety Devices using the example of a Centrifugal Fan:

  • Impeller speed
  • Shaft vibrations
  • Bearing temperature
  • Housing temperature
  • Fluid in the housing
  • Oil pump
  • Oil level and oil temperature in the tank
  • Oil pressure and oil flow
  • Oil cooler
  • Motor bearing temperature
  • Motor winding temperature

Compressor drives

Several types of motors are suitable as drive of a vapour recompressor. The selection of the drive depends on its efficiency and availability of the drive energy in each individual case.

Three-phase current asynchronous motor

The three-phase current asynchronous motor runs according to the number of its pole pairs at speeds of 3,000, 1,500, 1,000 or 750 rpm – in synchronism with the mains frequency – or – if a frequency converter is used – at infinitely variable speed.

The most important regulation variants for centrifugal fans and turbo-compressors are:

  • Speed control
  • Pre-rotation control
  • Suction pressure control
  • Bypass control


Existing Multi Stage Evaporator can be modified for heating by means of MVR. After the retrofitting measures, the existing effects are heated in parallel. For this purpose, the existing vapor ducts are dismantled, one or more mechanical recompressor are installed and new vapor ducts are installed. In this way, the energy flow in the plant is changed such that the major part of the evaporated vapor is now used to heat all effects. Only very small quantities of live steam and cooling water are required.


The costs for such a retrofitting will amortize within a short period of time.


abhinav chauhan
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